High-pressure discharge lamps usually include a tubular discharge vessel of transparent material, for example of transparent ceramic. The tubular discharge vessel is closed by at least one end plug of ceramic material fitted into the tubular discharge vessel, the plug being formed with an opening through which a current supply lead is sealed. An electrode including an electrode support rod and an electrode coil are secured to the current supply lead.
Sodium high-pressure discharge lamps are usually operated in saturated discharge conditions. During operation, only a portion of the fill in the discharge lamp, usually sodium and mercury, will vaporize. The remainder will condense in the form of a liquid amalgam at one or more positions in the discharge vessel at cold spots. In contrast to operation under unsaturated conditions, which is typical for mercury arc discharge lamps, the arc voltage depends highly on the operating conditions of the lamp, including for example ambient surrounding temperatures, supply voltages and the like. Due to the liquid amalgam, changes in cold-spot temperature feed back directly via variations in condensation and vaporization conditions to affect the density of the metal vapor within the lamp, and hence the arc voltage. The arc voltage, in turn, determines the lamp power when the lamp is operated, as is customary, with an inductance or choke. Positive feedback with respect to the cold-spot temperature will result. In operation of a lamp with a "constant wattage" ballast or auxiliary apparatus, positive feedback would not occur. The arc voltage would be affected only by ambient temperature.
Operation of lamps under unsaturated conditions has an advantage due to the high dependence of arc voltage on the operating conditions when the sodium vapor lamps are operated under saturated conditions. Some sodium high-pressure discharge lamps have been placed on the market in which the sodium loss in the discharge vessel has been reduced to such an extent that sufficient lifetime could be obtained without condensed sodium amalgam When using customary materials and production methods, however, for the discharge vessels, sodium loss is still too high that the buffering effect by the condensated sodium amalgam could be eliminated. Mercury, for all practical purposes, does not disappear from the lamp. Loss of sodium, however, causes constant shift of the sodium-to-mercury ratio in the direction of increased proportion of mercury. This shift is particularly high when the entire amalgam has vaporized and decreases when sufficient sodium can be supplied to the gas phase. This permits reduction of increase of arc voltage to a desired extent at a given sodium loss rate; the increase in arc voltage is derived from a change in the mol-relationships. The increase in sodium availability in the discharge vessel thus is capable of reducing the rise in arc voltage.
Two different solutions have been proposed to place the condensate within the discharge vessels.
One solution provides a cold spot outside of the ceramic tube, typically within the exhaust tube, see U.S. Pat. No. 3,723,784, Sulcs et al. The exhaust tube then has the character of an appendix. It is intended to obtain at least approximately reproducible cold-spot temperatures by suitable and careful shaping of this appendix. In this construction, the amalgam condensates at a point external to the surface defined by the ceramic tube. Such a construction has been given the term "external amalgam".
The other solution which has been proposed does not use an appendix but, rather, provides space within the ceramic tube behind the electrodes to collect the amalgam. It is, therefore, located within a surface defined by the ceramic tube, and the condensate in this position has also been referred to as an inner or interior amalgam, see German Patent 28 14 411 and European Published Application 0 074 188.
Customary designs for inner amalgam discharge vessels utilize a tubular ceramic discharge vessel into which a cylindrical ceramic plug having a smooth inner facing surface is fitted, and sealed by a glass solder or glass seal. The cylindrical ceramic plug is formed with a hole, concentric with the axis of the tube, through which a niobium tube or a niobium wire is carried to form the conductive connection to the electrode, as described for example in German Patent 28 14 411. In such a construction, only a small quantity of condensate can collect at the depressions which will then be formed at the ends of the tube and retained thereon by capillary forces even under conditions of vibration or shock. The quantity of amalgam which is necessary to buffer the sodium loss during the lifetime of the lamp usually is larger than that quantity which can be bound by capillary forces and, thus, renders the lamp sensitive with respect to mechanical shocks or other disturbances.
The location at which the actual arc starts on the electrode affects the construction of the inner amalgam discharge vessel. This undesired dependence of arc spot can be reduced if a direct sight line between the amalgam and the electrode is interrupted, see European Patent Application 0 074 188. It has been found particularly undesirable with respect to changes of the cold-spot temperature during the lifetime of the lamp if the discharge arc, upon ignition, can start where the condensate is located, or at the condensate. It may lead to spraying of the amalgam in the vicinity of the electrodes, to extended continued repeated starting of arcing at the amalgam and especially at its forward edge, and to at least partial operation, upon ignition, under half-wave operating conditions.
If the arc starts frequently at the amalgam, an additional disadvantage results: during the lifetime of the lamp, fissures can occur in the transition region from the plug to the ceramic tube based on mechanical damage to the discharge vessel caused by frequent arcing at the amalgam. Further, arcing from the amalgam results in substantial blackening of the discharge vessel tube in the vicinity of the electrodes. This blackening raises the cold-spot temperature and increases the arc voltage. Using a construction in accordance with the European Patent Application 0 074 188 achieves the goal of separation of potentials and renders interruption of the sight line between the electrode and the amalgam only partly effective. This lamp additionally appears to be sensitive to vibration or shock since the circular ring groove has a relatively high cross-sectional dimension.
The influences of the cold-spot temperature on lamp construction and lamp operation are important. These influences have been investigated for these reasons:
It is difficult to reach the arc operating voltage if, to improve the lifetime of the lamps, they are operated under partial loading, that is, in which the wall loading is decreased by increase of the inner diameter.
It is difficult to reach the required cold-spot temperature in sodium high-pressure lamps of less than 50 W rating with customary tubular discharge vessel construction. This difficulty increases as the power rating decreases. There is a definite need for a modified discharge vessel construction to enable obtaining higher cold-spot temperatures.
Lamps which have improved color rendition and which are shorter and have an increased diameter require substantially higher vapor pressure and hence a substantially higher cold-spot temperature than corresponding standard type lamps. It is customary to obtain this increase in temperature by use of heat damming or heat retaining sleeve structures.
Similar considerations apply to sodium high-pressure discharge lamps of the "plug-in" types which are intended to be interchangeable with similar mercury vapor high-pressure lamps without change of accessory or auxiliary apparatus. Such "plug-in" types usually use heat retention structures.
The cold-spot temperature can be influenced in the simplest way by changing the spacing between the tip of the electrode and the closing plug. Increasing the temperature by shortening this distance, however, has limits due to the geometric shape of the arc tubes, and especially if the rearward end of the electrode coil engages against the end of the current supply lead made of niobium. Higher cold-spot temperatures, without changing the ceramic tube construction, can then be obtained only by external heat retention structures, and particularly by heat shields described, for example, in U.S. Pat. No. 3,723,784, Sulcs et al. Assembling such heat shields to the lamp is expensive.